Research efforts of this laboratory center on documenting the organization and variation that exists in the brains of living vertebrates and utilizing these data to elucidate mechanisms of brain evolution. In this application, a plan is outlined for continuing studies of the organization and evolution of the vertebrate telencephalon. Living vertebrates constitute two major evolutionary radiations: jawless fishes (agnathans) and jawed vertebrates (gnathostomes), and thus far experiments on the telencephalon have been confined to gnathostomes. Studies carried out on amphibians and lungfishes (gnathostomes) during the past few years of this project have suggested several hypotheses regarding evolution of the vertebrate telencephalon that can be tested only by carrying out experiments on agnathans. It is therefore proposed to determine the connections of a number of telecephalic areas in both lampreys and hagfishes (the only living agnathans). Injections of horseradish peroxidase and tritiated proline will be made into appropriate brain areas to reveal the distribution of secondary olfactory tracts, dorsal thalamic projections to the telencephalon, and the efferents of those telencephalic areas that receive thalamic input. Data from these experiments will allow the testing of a major hypothesis derived from the past several year's studies: that telencephalic organization in lampreys (which differs from that in hagfishes) represents the primitive patterns for agnathans and is similar to that which existed in the earliest vertebrates, and that the pattern seen in hagfishes is, thus, derived. In addition, these experiments should establish whether telencephalic hypertrophy in hagfishes is due to olfactory specialization or to the independent evolution of non-olfactory laminated sensory areas. These studies will also provide the first data on brain organization in agnathans to compare with that which exists for gnathostomes and will yield considerable insights into mechanisms of brain evolution in vertebrates.